This application claims priority of Korea patent Application No. 2000-60950, filed on Oct. 17, 2000.
(a) Field of the Invention
The present invention relates to a shift control system of an automatic transmission and a method thereof, and more particularly, to a shift control system of an automatic transmission and a method thereof for increasing the output torque of an automatic transmission provided with a manual shift range in which a driver can manually select a shift range.
(b) Description of the Related Art
Generally, an automatic transmission of a vehicle determines a shift range according to vehicle speed and throttle valve opening on the basis of a shift pattern stored in a memory of a TCU (Transmission Control Unit).
However, in a vehicle equipped with an automatic transmission provided with a manual shift range in which a driver can manually select a shift range, when the vehicle is running in a heavy-load state such as running on a steep slope or towing a trailer, the oil temperature of the transmission is easily increased by frequent shifting owing to insufficient driving force and slip, and thereby output torque is further reduced.
For example, if a vehicle equipped with a shift lever operating in the order of P-R-N-D-4-3-2 is driven in a second speed range xe2x80x9c2xe2x80x9d of the manual shift range 4-3-2 when running on a steep slope or towing a trailer, an actual shift range of the transmission is not fixed at a second speed even though the shift lever is in the range xe2x80x9c2xe2x80x9d, but it can shift into a suitable speed between the first speed and the second speed corresponding to TCU (Transmission control unit) control according to a current driving state.
Such shifting into the first speed or the second speed is determined based on a predetermined shift pattern such as a 1⇄2 shift pattern shown in FIG. 6. In the 1⇄2 shift pattern of FIG. 6, a xe2x80x9c1xe2x86x922xe2x80x9d shift pattern represents a critical state in which the speed range is shifted into the second speed from the first speed, while a xe2x80x9c1←2xe2x80x9d shift pattern represents a critical state in which the speed range is shifted into the first speed from the second speed. The xe2x80x9c1xe2x86x922xe2x80x9d shift and xe2x80x9c1←2xe2x80x9d shift patterns are differentiated to avoid repeated shifting at a critical state. That is to say, there is hysteresis between the xe2x80x9c1xe2x86x922xe2x80x9d shift pattern and the xe2x80x9c1←2xe2x80x9d shift pattern. As can be seen in the xe2x80x9c1←2xe2x80x9d shift pattern in FIG. 6, according to the prior art, the speed range is shifted into the first speed only if the throttle valve opening reaches near the maximum value.
Therefore, when a vehicle is running in a heavy-load state such as when running on a steep slope or towing a trailer, the output torque from the transmission remains insufficient because the speed range remains in the second speed even when a throttle valve is almost wide open. Therefore the vehicle speed is reduced by lack of torque and the slip of friction elements in the automatic transmission possibly occurs in such a heavy-load state.
Additionally, the temperature of the transmission oil increases because of frequent shifts between the first speed and the second speed, because the driver repeats the process of completely depressing the accelerator pedal to shift the speed range into the first speed in order to produce enough driving torque, and releasing it to escape from the rapid increase of the driving speed, because speed range shifting into the first speed in a full throttle state will provide an abrupt increase of output torque.
Therefore, it is an object of the present invention to provide a shift control system of an automatic transmission and a method thereof for improving the driving force by down-shifting into the first speed according to a predetermined modified shift pattern when a condition for changing a shift pattern is met, and thereby preventing the oil temperature of the transmission from being excessively increased, when a vehicle provided with a manual shift range is running in a second speed in a heavy-load state.
To achieve the above objective, the present invention provides a shift control method of an automatic transmission provided with manual shift ranges comprising determining whether a vehicle driving state satisfies a predetermined condition for changing a shift pattern to a predetermined modified shift pattern when an accelerator pedal is operated while a vehicle is being driven in a second speed range xe2x80x9c2xe2x80x9d, and shifting into a first speed on the basis of the modified shift pattern if the vehicle driving state satisfies the predetermined condition for changing the shift pattern.
Preferably, the predetermined condition for changing the shift pattern is set as being that a sportiness index is greater than a predetermined value and a slope degree is greater than a predetermined value.
Said slope degree is calculated on the basis of a surplus driving force obtained by subtracting vehicle rolling resistance from an actual torque of an engine and an actual vehicle acceleration.
Preferably, a relevant portion of a 1←2. shift pattern of the modified shift pattern is formed with a curve such that the curve linearly rises from a first throttle valve opening at a first predetermined vehicle speed to a second throttle valve opening at a second predetermined vehicle speed, and the first predetermined vehicle speed and the second predetermined vehicle speed are respectively set as 20 km/h and 40 km/h, and the first throttle valve opening and the second throttle valve opening are respectively set as 20% and 60%.
Further, the predetermined condition for changing the shift pattern in order to increase the driving force is set as being that a vehicle speed is lower than a predetermined value and a throttle valve opening is greater than a predetermined value. The predetermined throttle valve opening is set as 20% to 60%, linearly corresponding with a vehicle speed of 20 km/h to 40 km/h.
The present invention also provides a shift control system for controlling an automatic transmission provided with manual shift ranges including a second speed range xe2x80x9c2xe2x80x9d, comprising a vehicle state detector for detecting a driving state of a vehicle, and a transmission control unit for determining whether the vehicle driving state satisfies a predetermined condition for changing a shift pattern to a predetermined modified shift pattern when an accelerator pedal is operated during driving in the second speed range xe2x80x9c2xe2x80x9d, and for generating a control signal to allow the shift range to shift into a first speed according to the modified shift pattern when the vehicle driving state satisfies the predetermined condition for changing the shift pattern.